1. Field of the Invention
The present invention relates to an improved method of making optical elements for microlithography, to optics made by the method and to electronic components and devices including the optics.
2. Related Art
Many electronic circuits are arranged in close proximity to each other in an electronic computer or controller. They are also called integrated circuits (IC) or chips. Electronic circuits are possible, which can perform many millions of circuit operations, by sophisticated arrangements. Because of the ever increasing computational and/or circuit functions of these circuits they are arranged in an ever-smaller space, i.e. their packing density is always becoming greater. According to the so-called Moore's law during controlled miniaturization of these circuits their computational power and/or processing speed doubles every 18 to 24 months. In order to achieve this it is necessary to continually make smaller transistors for this sort of processing and/or control circuit and to arrange them closer and closer above or next to each other. Only in this way miniaturization may permanently reduce the dissipated heat and the processing times.
Microlithography is used to make these types of miniaturized circuits. In microlithography a light-sensitive lacquer, a so-called photo resist, is illuminated by an expensive optical system with light. In this way images of previously designed conductor strips and circuits are produced on the photo lacquer. Further processing of these images produces an entire network of integrated circuits. The extent of the miniaturization of these circuits depends on the respective wavelengths of the light used during microlithography. Currently irradiating wavelengths in the UV, especially the deep UV range, are used in microlithography. For this purpose especially coherent light, particularly laser light, and of course from an excimer laser, such as a KrF laser with wavelengths of 248 nm, an ArF laser with wavelengths of 193 nm, and a F2 laser with a wavelength of 157 nm, can be used. In this way it is possible to make chip structures with a width of less than 100 nm.
However it is not only necessary to reduce the wavelength of the irradiating laser light to miniaturize chip structures, but also to improve the imaging precision of the entire irradiating optical system, i.e. the optical resolution of the system, must be improved.
From microscope engineering it is now known that the resolution of optics can be drastically improved by use of a so-called immersion fluid, which is arranged between the object being studied and the objective. Immersion attachments (e.g. Twinscan™ of ASML) have already been developed for commercially available lithography units and for fully functional lithography units, which however still contain conventional lens system.
Thus, for example, J. H. Burnett, et al, in “High Index Materials for 193 and 157 nm Immersion Lithography”, International Symposium on Immersion and 157 nm Lithography, International Symposium on Immersion and 157 nm Lithography, Vancouver, Aug. 2, 2004, describes the use of materials with a high index of refraction as materials for lens systems in Immersion photolithography. According to that reference the index of refraction should be substantially larger than that of water in order to obtain a maximum numerical aperture at a given wavelength. Thus Burnett, et al, recommends the use of the alkaline earth fluorides, CaF2, SrF2, and/or BaF2, which have an absolute refractive index of 1.5 to 1.58 at a wavelength of 193 nm and of 1.56 to 1.66 at 157 nm. Furthermore mixed crystals of the general formula BaxSr1- xF2 and BaxCa1-xF2 for x=0 to 1 and BaxCa1-xF2 with x about 0.1 are suggested for the optical elements. Also alkaline earth oxides, such as MgO, CaO, SrO and BaO, are proposed as possible lens materials. It is also possible to compensate the intrinsic birefringence by mixing these oxides according to this reference. Especially CaO can compensate the birefringence and at the same time increases the index of refraction.
However it has been shown that large-volume single crystals, which have the uniformity required for making this sort of lens, can be drawn from the melt only with difficulty with the suggested materials by means of known techniques. Furthermore the phase diagram for the composition, which is required to obtain a high index of refraction, is extremely critical and is characterized by a melting point that is reach only with difficulty by known crystal growing techniques. Finally no sufficient optical homogeneity is achieved with these mixtures.